This invention relates generally to semiconductor structures and manufacturing methods and more particularly to electrical components adapted for monolithic microwave integrated circuit (MMIC) fabrication.
As is known in the art, it is frequently desirable to fabricate, and electrically interconnect, both passive and active microwave components on a common substrate. Such arrangement is commonly referred to as a monolithic microwave integrated circuit. Typically the components are electrically interconnected with microstrip transmission lines. The microstrip transmission lines include strip conductor circuitry disposed on one surface of the structure and separated from a ground plane conductor by a substrate. The active devices are typically, for example, heterojunction bipolar transistors (HBTs), field effect transistors (FETs), bipolar devices or PIN diodes. A common substrate material used is gallium arsenide. Such substrate material is suitable for the passive microstrip transmission line circuitry, the support of passive devices, such as capacitors, and also for formation of single crystal epitaxial layers used to form the active semiconductor region for the HBTs and the FETs.
As is known in the art, many active devices are formed with interdigitated electrodes. For example, FETs (or bipolar transistors) adapted to operate at high frequencies are sometimes formed with finger shaped gate electrodes (or base electrodes) and finger shaped drain electrodes (or collector electrodes). The finger shaped electrodes are disposed in an interdigitated relationship over a surface of a semiconductor body. Source electrodes (or emitter electrodes) are disposed over the surface and are positioned between a pair of the gate electrodes (or base electrodes). The gate electrodes (or base electrodes) are electrically connected, at proximal ends thereof, to a bus disposed on the surface of the semiconductor. Likewise, the drain electrodes (or collector electrodes) are electrically connected, at proximal ends thereof, to a bus disposed on the surface of the semiconductor body. The source electrodes are connected using bridging conductors, sometimes referred to as air-bridges, which have ends connected to a pair of the source electrodes and which span over gate and drain electrodes. The latter technique is described in U.S. Pat. No. 4,456,888, issued Jun. 26, 1984 and entitled "Radio Frequency Network Having Plural Electrically Interconnected Field Effect Transistor Cells".